Flow cytometry is one of the most widely used and indispensable techniques for analysis of cell phenotypes and for isolation of specific cell populations. The Flow Cytometry Core resources supported by the POl provide the Program's investigators with increased access in a cost effective way to instruments for analytical flow cytometry, which in some cases must be maintained in appropriate biosafety containment condifions. The core has three specific aims: 1) Provide and maintain the necessary research infrastructure needed for flow cytometry studies, with appropriate biosafety containment safeguards;2) Provide technical assistance in the performance and interpretafion of established techniques involving flow cytometry;3) Introduce new and update existing technologies to maintain cutting edge capabilifies. The core facility supported by the POl has full responsibility for maintaining one analytical flow cytometer, and contributes to the maintenance and operation of one high speed cell sorter. Access through this core to FACS analysis and sorting techniques under Biosafety Level (BSL)-2 and BSL-3 condifions has played an essential role in research funded by this POl grant during the past 5 years, and will continue to do so into the next funding period. The high cost ofthe instrumentafion and the need for skilled personnel makes the establishment of a core facility for flow cytometry the only pracfical approach to management of these resources. In addition, this core facility provides the only possibility for the POl investigators to do flow cytometry in a BSL-3 environment within the institution, and thus is a crucial facility for some of the studies that are ongoing or planned within the context of the program. The core also provides an environment in which biosafety containment procedures can be stricfiy overseen and enforced, thus ensuring the safety of personnel using flow cytometry techniques in their work with potentially dangerous infectious agents. The analytical capabilifies of the core allow the program investigators to identify and characterize cell populations of interest in a wide range of studies related to mycobacterial pathogenesis and the host immune response. RELEVANCE (See instructions): This proposal requests support for a core facility that will provide sophisficated equipment to allow investigators in the POl Program to analyze cell populations for their expression of various molecules. The methods that will be supported are of crucial importance to many types of experiments that will be performed to address the overall goal of designing and constructing better vaccines and drugs to prevent tuberculosis.